The invention relates to a method of controlling heat exchangers in order to set a controlled variable on the process side of the heat exchanger.
The invention resides in the field of chemical engineering operations wherein a controlled energy supply is required. On a commercial scale, these energy consumers are generally supplied centrally from a power plant. The steam produced by the power plant is conveyed via a so-called steam line to the individual consumers. At the consumer end, in exceptional cases the incoming steam is fed directly to the process. Mainly, however, only the heat contained in the steam is transmitted to the process via an interposed heat exchanger. The control of the transmitted quantity of energy forms the object of the present invention.
The method customarily used on a commercial level can be described making reference to FIG. 1. In order, for example, to control the temperature in a process (TC), the actual correcting variable, the transmitted energy, is converted into a substitute correcting variable, namely the quantity of steam fed into the heat exchanger 3 followed by steam trap 4. This is illustrated in the drawing by the feed control unit FC which controls the valve 2. The control value employed by the feed control unit is generally the pressure drop .DELTA..sub.p in the steam across an orifice gauge 5 within the feed line (FIG. 2). Under ideal conditions, i.e. in the case of constant pressure and constant temperature, the volume flow, and thus also the mass flow through the orifice gauge, is proportional to the root of the pressure drop across the orifice gauge (orifice equation).
However, the disadvantage of this known method consists in that changes can occur in the conditions (pressure, temperature) inside a steam line supplying a plurality of consumers from the power plant. For example, even the switching on and off of consumers can give rise to disturbances along a steam line.